Friction clutch with adjusting device

ABSTRACT

A friction clutch with a back-pressure plate, which is connected to a housing so as to rotate with it, and a pressure plate, which is received by leaf springs such that it can be displaced axially and is fixed with respect to the back-pressure plate so as to rotate with it and which is prestressed by a disc spring to a preset operating point with friction linings clamped in between, and an adjusting device, for adjusting a changing operating point by a ramp ring which is distributed in the circumferential direction and over the circumference, is received on the pressure plate on complementary mating ramps of the pressure plate and is rotated by a spindle device depending on a change in the operating point, and a sensing device which detects a change in the operating point and controls the spindle drive.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is filed under 35 U.S.C. §120 and §365(c) as acontinuation of International Patent Application No. PCT/DE2011/001939filed Nov. 7, 2011 and claims priority of German Patent Application No.10 2010 052 021.7 filed Nov. 19, 2010, German Patent Application No. 102011 081 476.0 filed Aug. 24, 2011, and German Patent Application No. 102011 081 475.2 filed Aug. 24, 2011, which applications are incorporatedherein by reference to their entireties.

FIELD OF THE INVENTION

The invention relates to a friction clutch that includes acounter-pressure plate connected to a housing in a rotationally fixedway and a pressure plate received to be rotationally fixed and axiallydisplaceable relative to the counter-pressure plate by means of leafsprings and pre-tensioned to a pre-set operating point by a diaphragmspring while clamping friction linings, as well as a readjustment devicefor readjusting a changing operating point by means of a ramp ringreceived on complementary counter-ramps of the pressure plate by rampsthat are distributed in the circumferential direction along thecircumference, the ramp ring being rotated by a spindle drive as afunction of a change of the operating point, as well as a sensing devicedetecting a change of the operating point and controlling the spindledrive.

BACKGROUND OF THE INVENTION

German Patent Specification DE 10 2008 051 100 A1 discloses a clutchdevice. In this friction clutch, an operating point that changes due towear on the friction linings is automatically readjusted. A spindledrive is provided on the pressure plate to rotate the ramp ring in onedirection in the case of wear. For this purpose, a pawl is fixedlyarranged on the housing. When a predetermined amount of wear isexceeded, the pawl rotates a pinion of the spindle drive before thefriction clutch is disengaged by creating a form-locking connectionbetween the pinion and the pawl. As the friction clutch is beingdisengaged, the operating point is corrected by a rotation of the pinioncaused by the pawl. This method only provides discontinuous step-by-stepcompensation of the friction lining wear.

German Patent Application No. 10 2010 052 021.7 furthermore discloses afriction clutch with a readjustment device wherein the sensing andactuating device consisting of a pawl and a pinion is replaced by a wraprope connected to the housing and wrapped around the spindle of thespindle drive. The wrap rope rotates the spindle of the spindle drive asa function of the wear on the lining to correct the operating point,which has changed due to lining wear, in a continuous and infinitelyvariable way. An operating point that changes in both directions, forexample in one direction due to lining wear and in the other directiondue to swelling friction linings and/or potting of the pressure plateand/or counter-pressure plate, cannot be completely corrected.

BRIEF SUMMARY OF THE INVENTION

A friction clutch with a back-pressure plate, which is connected fixedlyto a housing so as to rotate with it, and a pressure plate, which isreceived by leaf springs such that it can be displaced axially and isfixed with respect to the back-pressure plate so as to rotate with itand which is prestressed by a disc spring to a preset operating pointwith friction linings clamped in between, and an adjusting device, foradjusting a changing operating point by a ramp ring which is distributedin the circumferential direction and over the circumference, is receivedon the pressure plate on complementary mating ramps of the pressureplate and is rotated by a spindle device depending on a change in theoperating point, and a sensing device which detects a change in theoperating point and controls the spindle drive. To compensate fordisplacements of the operating point in both directions in an infinitelyvariable manner, the spindle drive is received fixedly on the housingand contains three shaft sections which are arranged concentrically withrespect to one another and can be rotated to a limited extent withrespect to the housing and with respect to one another, wherein theinner shaft section has a spindle mechanism, which drives the ramp ring,and can be rotated firmly in one rotational direction with respect tothe middle shaft section and can be rotated to a limited extent in theother rotational direction counter to the action of first energy stores,and the middle shaft section and the outer shaft section which containsthe sensing device with respect to the ramp ring are arranged such thatthey are secured against rotation with respect to the housing in eachcase by two switchable wraparound belts which are connected counter toone another.

An object of the invention is to refine a friction clutch withreadjustment device wherein an operating point that changes in bothdirections can be continuously corrected in an infinitely variable way.

In accordance with the invention, this object is attained by a frictionclutch that includes a counter-pressure plate connected to a housing soas to be fixed against rotation relative thereto and a pressure platereceived to be fixed against rotation and axially displaceable relativeto the counter-pressure plate by means of leaf springs and pretensionedto a pre-set operating point by a diaphragm spring while clampingfriction linings, as well as a readjustment device for readjusting achanging operating point by means of a ramp ring received on thepressure plate by ramps that are distributed in the circumferentialdirection along the circumference and rest on complementarycounter-ramps on the pressure plate, the ramp ring being rotated by aspindle drive as a function of a change of the operating point, as wellas a sensing device detecting a change of the operating point andcontrolling the spindle drive, wherein the spindle drive is fixedlyreceived on the housing and includes three shaft portions arranged to beconcentric with each other and rotatable relative to the housing andrelative to each other to a limited extent, the inner shaft portioncontaining a spindle drive that drives the ramp ring and arranged so asto be fixed in one direction of rotation relative to the central shaftportion and rotatable to a limited extent against the action of firstenergy storage elements in the other direction of rotation, and each ofthe central and outer shaft portion, which latter contains the sensingdevice towards the ramp ring, being respectively arranged to be securedagainst rotation relative to the housing by means of two respectiveswitchable wrap belts arranged to act in opposite directions.

Due to the fact that the spindle drive is received on the housing,insulation thereof relative to vibrations of the pressure plate isachieved, thus at least minimizing vibration-induced erroneousreadjustments.

Due to the fact that the switchable wrap belts or wrap ropes arearranged to act in opposite directions, the shaft portions are held in away to be secured against rotation, thus stabilizing a predeterminedoperating point or a corrected operating point after a readjustment. Thespindle itself may be arranged to be secured against rotation in that itis arranged to be secured against rotation relative to the second shaftportion in one direction and rotatable against the action of energystorage elements such as leaf springs in the other direction. With thesecond shaft portion correspondingly secured against rotation, thespindle of the spindle drive may automatically rotate in one directionof rotation, a rotation in this direction of rotation being precludeddue to the interlocking of ramp ring and pressure plate due to thediaphragm spring when the friction clutch is engaged. The rotation ofthe spindle thus depends on the rotatability of the second shaftportion. A first pair of wrap belts that are arranged to act in oppositedirections and wrapped around the central shaft portion may be arrangedto be fixed relative to the housing by one end and to be elastic by theother end due to an energy storage element that is connected in-between.Due to this arrangement, a rotation of the second portion is possibleonly when one of the two wrap belts is switched off. The control thereofoccurs by the third shaft portion against the action of the energystorage elements of these wrap belts. For this purpose, a second pair ofwrap belts that are arranged to act in opposite directions may befixedly connected to the third shaft portion by one end and elasticallyconnected to the housing by the other end with an energy storage elementsuch as a leaf spring connected in-between and may be wrapped around thecentral shaft portion.

The central shaft portion is controlled by the third shaft portion bymeans of the sensing device arranged between the ramp ring and the thirdshaft portion to detect the need for readjustment when the pressureplate leaves a predetermined stroke range. A need for readjustment inthe case of an operating point shifting towards the counter-pressureplate, for example, due to lining wear of the friction linings isdetected when the friction clutch is engaged and stored in an energystorage element. When the friction clutch is disengaged, the operatingpoint is corrected by a corresponding rotation of the first shaftportion and thus of the spindle that rotates the ramp ring in acorresponding way. An operating point that has shifted in the directionof the diaphragm spring, for example, due to swelling friction liningsand/or potting of clutch components is detected by the sensing devicewhen the friction clutch is being disengaged or in the disengaged stateso that the operating point may be corrected without buffering as aresult of a lack of blocking of the ramp ring.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature and mode of the proposed friction clutch with readjustmentdevice will be explained in more detail in the following detaileddescription of the invention taken with the accompanying drawingfigures, in which:

FIG. 1 is a partial sectional view of a friction clutch withreadjustment device;

FIG. 2 is a diagrammatic three-dimensional view of a functional example;

FIG. 3 is a diagrammatic side view of a functional example;

FIG. 4 a is a view of the first and second shaft portions in afunctional condition;

FIG. 4 b is a view of the first and second shaft portions in anotherfunctional condition; and,

FIG. 4 c is a view of the first and second shaft portions in yet anotherfunctional condition.

DETAILED DESCRIPTION OF THE INVENTION

At the outset, it should be appreciated that like drawing numbers ondifferent drawing views identify identical, or functionally similar,structural elements of the invention.

While the present invention is described with respect to what ispresently considered to be the preferred aspects, it is to be understoodthat the invention as claimed is not limited to the disclosed aspects.

Furthermore, it is understood that this invention is not limited to theparticular methodology, materials and modifications described and, assuch, may, of course, vary. It is also understood that the terminologyused herein is for the purpose of describing particular aspects only,and is not intended to limit the scope of the present invention, whichis limited only by the appended claims.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to one of ordinary skill inthe art to which this invention belongs. Although any methods, devicesor materials similar or equivalent to those described herein can be usedin the practice or testing of the invention, the preferred methods,devices, and materials are now described.

FIG. 1 is a partial sectional view of a friction clutch 1 arranged aboutan axis of rotation 2 and including a counter-pressure plate 3 fixedlyconnected to a housing 4. In the housing 4, the pressure plate 5 isreceived as to be fixed rotationally and axially displaceable relativeto the counter-pressure plate 3 by means of non-illustrated leaf springsand is urged against the counter-pressure plate 3 of the pressure plate5 by the diaphragm spring 6 supported on the housing 4. Between frictionsurfaces 9, 10 of the counter-pressure plate 3 and the pressure plate 5,friction linings 7 of the clutch disc 8 are clamped and in a frictionalengagement therewith, thus forming the operating point, which may bedefined as an axial position of the friction surface relative to theaxis of rotation 2. If the pre-tensioning of the diaphragm spring 6 isreduced by a non-illustrated disengagement system, the frictionalengagement is released due to a shifting of the pressure plate 5 as aresult of the effect of the leaf springs that are axially pre-tensionedin the engaged condition of the friction clutch.

The friction clutch 1 includes the readjustment device 11 that isself-adjusting in the case of a changing operating point. Thereadjustment device 11 includes the spindle drive 12 received on thehousing 4 and the ramp ring 13 arranged between the diaphragm spring 6and the pressure plate 5. The ramp ring 13 has rising ramps 14distributed about the circumference and facing the pressure plate 5.Rising ramps 14 rest on counter-ramps 15 of the pressure plate 5 so thatwhen the ramp ring 13 rotates, the distance between the friction surface10 and the diaphragm spring 6 increases or decreases, depending on thedirection of rotation. Thus, the operating point may be changed relativeto the diaphragm spring, and when the operating point shifts due to wearor swelling of the friction linings or as a result of other variablessuch as potting of the pressure plate 5, the operating point may bechanged in such a way that the diaphragm spring may maintain itspredefined angle of engagement relative to the ramp ring 13, thusmaintaining the transmission of power predefined by the release stroke.

To detect a changing operating point, the sensing device 16 is providedbetween the ramp ring 13 and the spindle drive 12. In FIG. 1, thesensing device 16 is formed by a sensing element 17 provided on thespindle drive 12 and by the two stops 18, 19 provided on the ramp ring13 with clearance relative thereto. With the operating point unchanged,during the operating processes of the friction clutch, the sensingelement 17 is located between the stops 18, 19 shifted back and forth.When the operating point changes, in the disengaged and in the engagedstate, respectively, one of the stops 18, 19 impinges on the sensingelement 17 and displaces the latter to correct the changed operatingpoint.

For example, if the operating point shifts in the direction of thecounter-pressure plate 3 due to lining wear, in the engaged state thesensing element17 impinges on stop 19 and a need for readjustment isregistered. In the case of a swelling of the friction linings 7 and/orpotting of the pressure plates 5, the operating point shifts in thedirection of the diaphragm spring 6 and when the friction clutch isdisengaged the sensing element 17 impinges on stop 18 and a need forreadjustment is registered. It is to be understood that the releasesystem of the friction clutch is designed for a constant stroke and iscorrespondingly calibrated and continuously readjusted.

FIG. 2 is a three-dimensional view of a functional model of thereadjustment device 11 to illustrate the structure and functioning ofthe spindle drive 12 when a need for readjustment has been registered.The spindle drive 12 contains three shaft portions 20, 21, 22 arrangedto be concentric with each other. The inner shaft portion 20 containsthe spindle in a non-illustrated way and a spindle nut arranged thereonto be rotationally fixed and to be axially displaceable in the case of arotation of the spindle. The spindle nut engages the ramp ring 13 in aform-locking way and rotates the latter in a clockwise orcounter-clockwise direction, depending on the direction of rotation ofthe spindle, thus being capable of correcting the operating point inboth axial directions. The inner shaft portion 20 is received forrotation in the housing 4 of the friction clutch 1 (FIG. 1). In the formof a hollow shaft the central shaft portion 21 is received on shaftportion 20 to rotate to a limited extent.

As becomes apparent from FIG. 2 and in more detail from FIGS. 4 a to 4c, shaft portions 20, 21 are rotationally coupled to each other. In onedirection of rotation, the connection is inelastic by means of the stops23 and in the other direction of rotation, the connection is elastic bymeans of the energy storage elements 24 such as leaf springs. Thus, if,in the illustrated example, shaft portion 20 is entrained by shaftportion 21 in a counter-clockwise direction, (FIG. 4 c), the rotation istorsionally stiff and occurs without buffering of the rotational travelto correct the operating point; with the friction clutch disengaged, thespindle, which is formed on the shaft portion as an axial extension, forexample, immediately rotates the then released ramp ring via the spindlenut. When shaft portion 21 is rotated with the friction clutch closed,for example, if the friction lining thickness has decreased, shaftportion 20 is blocked by the spindle and the axially loaded ramp ring 13(FIG. 2), causing shaft portions 20, 21 to be rotated against the actionof the energy storage elements 24. Thus, the rotational travel isbuffered until the ramp ring 13 is released upon a subsequentdisengagement of the friction clutch and the energy storage elements 24drive shaft portion 20 and subsequently, the spindle, spindle nut, andramp ring 13, thus effecting a correction of the operating point.

As further becomes apparent from FIG. 2, shaft portion 21 is securedagainst rotation by means of a wrap belts 25, 26 that form a pair andare arranged to act in opposite directions. One end 27, 29 of each wrapbelt 25, 26 is fixedly connected to the housing 4 and the other end 28,30 is elastically connected by means of the energy storage elements 31,32, which in the illustrated example are leaf springs 33, 34 connectedto the housing 4 on one side. The wrap belts 25, 26 arranged about shaftportion 21 pull tight in opposite directions and block the rotarymovement of shaft portion 21 and are loosened in the other direction ofrotation as soon as the pre-tensioning of the leaf springs 33, 34 isreleased.

In the case of a readjustment, this torque is transmitted in adirection-of-rotation-specific way from shaft portion 22 to shaftportion 21 by means of the wrap belts 35, 36 that are arranged to act inopposite directions. For this purpose, one end 37, 39 of each of thewrap belts 35, 36 wrapped around shaft portion 21 is fixedly connectedto shaft portion 22 whereas the other end 38, 40 is elasticallyconnected by means of the energy storage elements 41, 42, which in theillustrated example leaf are springs 43, 44 connected to the housing 4on one side. Shaft portion 22 includes the sensing element 17, whichextends into the opening 45 with stops 18, 19.

The pre-tensioning of the wrap belts 25, 26, 35, 36 is released in atargeted way by means of the ramp ring 13 as a function of the latter'smovement perpendicular to the axis of rotation of the shaft portions 20,21, 22. For this purpose, stops 46, 47 are provided on the ramp ring 13.In respective pairs, stops 46, 47 release the pre-tensioning of the leafsprings 33, 34, 43, 44 as a function of the direction of movement of theramp ring 13, thus eliminating the wrapping effect of the wrap belts 25,26, 35, 36.

During normal operation of the friction clutch 1 (FIG. 1) withoutreadjustment the sensing element 17 moves between the stops 18, 19 ofthe ramp ring and no readjustment is made. Due to the fact that shaftportion 21 is locked in both directions of rotation, shaft portion 20remains positioned relative thereto by means of the stops 23 and energystorage elements 24 (FIG. 4 a); the spindle does not rotate and noreadjustment of the ramp ring 13 occurs.

When the ramp ring 13 with the sensing element 17 moves as a result offriction lining 7 wear (FIG. 1) and thus of a shifting of the pressureplate 5 (FIG. 1)in the direction of stop 19 in the engaged state of thefriction clutch 1, the sensing element 17 is entrained by stop 19. Inthe process, the wrap belt 35 pulls and, releasing wrap belts 25, 36 bymeans of stop 47, entrains shaft portion 21 in a counter-clockwisedirection. As a result of the fact that the ramp ring 13 is clampedbetween the diaphragm spring 6 and the pressure plate 5 (FIG. 1) whenthe friction clutch 1 is engaged and the fact that the spindle isprevented from rotating on shaft portion 20, shaft portion 21 rotatesrelative thereto against the action of the energy storage elements 24(FIG. 4 b). Once the friction clutch 1 has been disengaged, thepre-tensioning of the ramp ring 13 by the diaphragm spring 6 is releasedand, shaft portion 20 rotates relative to shaft portion 21 in acounter-clockwise direction thereby reducing the pre-tensioning of theenergy storage elements 24 (FIG. 4 c) and the spindle rotates the rampring 13 in the circumferential direction to correct the changedoperating point.

In the reverse case, for example, due to potting of the pressure plate 5and/or swelling friction linings 7, the sensing element 17 moves in thedirection of stop 18 and is entrained by stop 18 when the frictionclutch 1 is being disengaged. In the process, wrap belts 26, 35 areloosened by stop 46 and shaft portion 21 is entrained in a clockwisedirection by wrap belts 25, 36. As the ramp ring 13 is released when thefriction clutch 1 is being disengaged, a buffering of the rotarymovement may be dispensed with and the rotation of shaft portion 20 byshaft portion 21 may occur directly by stops 23 without an elasticityconnected in-between, causing the ramp ring 13 to be driven by thespindle.

FIG. 3 is a diagrammatic representation of the friction clutch 1 withthe readjustment device 11. On the one hand as it is rotated, the rampring 13, which is received between the pressure plate 5 and thediaphragm spring 6 received on the housing 4, corrects a changingoperating point by changing the distance between the pressure plate 5and the diaphragm spring 6 by means of the ramps 14. On the other hand,the ramp ring 13 forms the sensing device 16 for sensing the need forreadjustment when the operating point has changed. For this purpose, theramp ring 13 includes stops 18, 19, which move relative to the sensingelement 17 of the spindle drive 12.

In the illustrated exemplary embodiment, stop 19 impinges on the sensingelement 17, which means that the operating point starts to shift towardscounter-pressure plate 3 (FIG. 1) and away from the diaphragm spring 6due to a reduced friction lining thickness. As a result, wrap belts 25,36 begin to loosen due to stop 47 of the ramp ring 13, whereas theactive wrap belts 26, 36 transmit the rotary movement of shaft portion22 enforced by the ramp ring 13 via the sensing element 17 to shaftportion 21. As a result of the fact that the friction clutch 1 isengaged, shaft portion 20, which is kinematically coupled to the rampring 13 via the spindle and the spindle nut, is blocked and preventedfrom rotating. With increasing lining wear, a relative rotation betweenshaft portions 20, 21 is build up via the energy storage elements 24.When the friction clutch 1 is being disengaged and the ramp ring 13 isreleased, relative rotation is reduced as the ramp ring 13 is rotatedand the operating point is corrected until stops 23 limit the relativerotation of shaft portions 20, 21.

Thus, it is seen that the objects of the present invention areefficiently obtained, although modifications and changes to theinvention should be readily apparent to those having ordinary skill inthe art, which modifications are intended to be within the spirit andscope of the invention as claimed. It also is understood that theforegoing description is illustrative of the present invention andshould not be considered as limiting. Therefore, other embodiments ofthe present invention are possible without departing from the spirit andscope of the present invention.

REFERENCE NUMERALS

-   1 friction clutch-   2 axis of rotation-   3 counter-pressure plate-   4 housing-   5 pressure plate-   6 diaphragm spring-   7 friction lining-   8 clutch disc-   9 friction surface-   10 friction surface-   11 readjustment device-   12 spindle drive-   13 ramp ring-   14 ramp-   15 counter-ramp-   16 sensing device-   17 sensing element-   18 stop-   19 stop-   20 shaft portion-   21 shaft portion-   22 shaft portion-   23 stop-   24 energy storage element-   25 wrap belt-   26 wrap belt-   27 end-   28 end-   29 end-   30 end-   31 energy storage element-   32 energy storage element-   33 leaf spring-   34 leaf spring-   35 wrap belt-   36 wrap belt-   37 end-   38 end-   39 end-   40 end-   41 energy storage element-   42 energy storage element-   43 leaf spring-   44 leaf spring-   45 opening-   46 stop-   47 stop

What is claimed is:
 1. A friction clutch (1), comprising: acounter-pressure plate (3) connected to a housing (4) in a rotationallyfixed way; a pressure plate (5) received to be rotationally fixed andaxially displaceable relative to said counter-pressure plate (3) bymeans of leaf springs and pre-tensioned to a pre-set operating point bya diaphragm spring (6); clamping friction linings (7) received betweensaid pressure plate (5) and said diaphragm spring (6); a readjustmentdevice (11) for readjusting a changing operating point by means of aramp ring (13) received between said pressure plate (5) and saiddiaphragm spring (6) by means of ramps (14) distributed in thecircumferential direction and along the circumference resting oncomplementary counter-ramps (15) of said pressure plate (5), said rampring (13) being rotated by a spindle drive (12) as a function of achange of said operating point; and, a sensing device (16) detecting achange of said operating point and controlling said spindle drive (12),wherein said spindle drive (12) is fixedly received on said housing (4)and includes three shaft portions (20, 21, 22) arranged to be concentricwith each other and rotatable relative to said housing (4) and relativeto each other to a limited extent, an inner shaft portion (20)containing a spindle drive that drives said ramp ring (13) and arrangedrelative to a central shaft portion (21) in a way to be fixed in onedirection of rotation and rotatable to a limited extent against theaction of first energy storage elements (24) in the other direction ofrotation, and said central and an outer shaft portion (21, 22), whichcontains said sensing device (16) towards said ramp ring (13), beingrespectively arranged to be secured against rotation relative to saidhousing (4) by means of two respective switchable wrap belts (25, 26,35, 36) arranged to act in opposite directions.
 2. The friction clutch(1) recited in claim 1, wherein a first pair of wrap belts (25, 26)arranged to act in opposite directions is fixedly connected to saidhousing (4) by an end (27, 29) and elastically connected to said housing(4) by another end, with an energy storage element (31, 32) connectedin-between, and is wrapped around said central shaft portion (21). 3.The friction clutch (1) recited in claim 2, wherein a second pair ofwrap belts (35, 36) arranged to act in opposite directions is fixedlyconnected to the third shaft portion (22) by an end (37, 39) andelastically connected to said housing (4) by another end (38, 40), withan energy storage element (41, 42) connected in-between, and is wrappedaround said central shaft portion (21).
 4. The friction clutch (1)recited in claim 2, wherein said energy storage elements (31, 32) areleaf springs (33, 34).
 5. The friction clutch (1) recited in claim 3,wherein said energy storage elements (41, 42) are leaf springs (43, 44).6. The friction clutch (1) recited in claim 1, wherein when detectedwith said friction clutch (1) in an engaged condition, a shifting ofsaid operating point in a direction of said counter-pressure plate (3)is stored in said first energy storage element (24) and said operatingpoint is corrected in a disengagement process of said friction clutch.7. The friction clutch (1) recited in claim 1, wherein a shifting ofsaid operating point in a direction of said diaphragm spring (6) in adisengagement process of said friction clutch (1) is corrected withoutintermediate storage.
 8. The friction clutch (1) recited in claim 1,wherein a stop (18, 19) arranged with clearance is provided between saidramp ring (13) and said third shaft portion (22) in both directions ofsaid shifting operating point.
 9. The friction clutch (1) recited inclaim 8, wherein an amount of said clearance between said stops (18, 19)of said ramp ring (13) and of said third shaft portion (22) correspondsto a stroke of said friction clutch (1) between engaged and disengagedconditions.